1. Technical Field of the Invention
The present invention relates to a gear pump used to lubricate an engine (an internal combustion engine), and particularly to a gear pump capable of reducing the engine load at a high RPM rate of the engine.
2. Prior Art
A gear pump driven by an engine capable of reducing engine load at high RPM (revolutions per minute) rates of the engine is disclosed in Examined Japanese Utility Model Publication 5-33739. As shown in FIG. 14, this gear pump for lubricating an engine comprises: a first pump formed of a driving gear 1; a first driven gear 2 externally contacting and engaging with said driving gear 1; a casing including a second pump formed of a second driven gear 3 externally contacting and engaging with the driving gear 1; an outlet circuit 6 connected to an outlet port 5 of the second gear pump which is formed of the driving gear 1 and the second driven gear. 3 and included in the casing; and an outlet circuit 7 which is connected to a port 4 of the first pump formed of the driving gear 1 and the first driven gear 2, and connected through a non-return valve 8 to the outlet circuit 6. The prior art pump also has: a bypass circuit 9 which is branched from the outlet circuit 6 between the outlet port 4 and the non-return valve 8 and is connected to an oil pan; a flow regulation valve 10 which is provided in said bypass circuit 9, fully closed at a lower limit lubricatable pressure to fully close the bypass circuit 9, and opened at an increased pressure of a lubricant to open the bypass circuit 9; and a pressure communicating circuit 12 which communicates from an outlet circuit 11 connected to the outlet circuit 7 connecting the outlet circuit 6 to the flow regulation valve 10. There is also a temperature sensing valve 13 which is provided in said pressure communicating circuit 12 that fully closes the pressure communicating circuit 12 at an appropriate temperature of the lubricant, and that opens the pressure communicating circuit 12 at an increased temperature of the lubricant.
With the above-mentioned assembly, the gear pump operates as described below. The second gear pump formed of the driving gear 1 and the second driven gear 3 is operated at a revolution speed in proportion to the revolution speed of an engine, and supplies the lubricant through the outlet circuit 6 and the outlet circuit 11 to places requiring the lubrication of the engine at all times. When the temperature and pressure of the lubricant is low, the first gear pump formed of the driving gear 1 and the first driven gear 2 supplies all the discharged lubricant, which passes from the port 4 through the outlet circuit 7 and pushes up the non-return valve 8 to join the outlet circuit 6, through the outlet circuit 11 to places requiring the lubrication of the engine in a manner similar to the above.
When the lubricant temperature increases, the temperature sensing valve 13 operates to cause the pressure communicating circuit 12 to be opened. Consequently, the lubricant in the outlet circuit 11 acts on the valve disk of the flow regulation valve 10. In this case, when the lubricant pressure is within the lower lubricant limit pressure, the valve disk does not operate. This means that the bypass circuit 9 is fully closed to cause all the discharged oil to be supplied to the engine. When the lubricant pressure exceeds the lower lubricant limit pressure, part of the discharged oil is returned through the bypass circuit 9 to the oil pan. When the lubricant pressure is high, the flow regulation valve 10 is fully opened to cause the bypass circuit 9 to be fully opened.
This causes the lubricant in the outlet circuit 7 to be bypassed. Consequently, the lubricant pressure is reduced to an extent that the pressure becomes lower than the lubricant pressure in the outlet circuit 6. Therefore, the non-return valve 8 does not operate to cause all the discharged oil of the first pump to be bypassed.
In this way, in Examined Japanese Utility Model Publication 5-33739, the discharge rate of the first pump is regulated by both the lubricant temperature and the lubricant pressure at all times, so that a lubricant exceeding a lubricant pressure and a lubricant temperature required at respective times is continuously bypassed. Therefore, no extra work is performed, thereby reducing an engine load.
However, the gear pump as shown in the above-mentioned Examined Japanese Utility Model Publication 5-33739, there are problems as shown below:
(1) The above-mentioned outlet ports 4, 5, outlet circuits 6, 7, 11, bypass circuit 9 and the like together with inlet ports and inlet circuits are formed in the casing, and the above-mentioned non-return valve 8 and flow regulation valve 10 are housed in the casing, which causes the composition of the gear pump to become very complex, the degree of freedom in design and manufacture to become narrow, and the manufacturing cost to be increased.
(2) Even if a ball valve is used for the above-mentioned non-return valve, a guide cylinder having an opening or a slit is provided in a manner to surround the ball valve in order to make smooth the engaging with and disengaging from the valve seat, where the guide cylinder is disposed in a space in which a passage is changed perpendicularly from the vertical to horizontal direction, the ball valve is pushed against the horizontal passage side due to the flow of oil and the like, so that a larger opening causes the ball valve to be bound to the opening and thus to be prevented from a smooth operation.
The fine slit also causes the fluid resistance to become large.
(3) The valve piston of the above-mentioned flow regulation valve opens/closes the output ports to have a large slide stroke, so that the lubrication on the outer periphery of the valve piston is poor so as to cause scuffing and valve malfunction.